A helmet is being widely used in order to protect the head of an occupant of a motorcycle or a worker in a construction site. In most cases, in these environments, people are surrounded by various kinds of noise. For example, the occupant of the motorcycle is exposed under various types of noise environments such as engine noise, exhaust noise and road noise in a relatively high frequency band, and furthermore wind noise in a relatively low frequency band. For that reason, a helmet equipped with a system is proposed which collects an external sound such as noise from a microphone arranged nearby an ear of a user and generates a sound having a reverse phase to the external sound from a speaker to actively attenuate the external sound as exemplified in patent documents 1 and 2.
However, actually, the noise includes various frequency bands, a noise attenuation effect is effectively exerted for a target band, but the noise cannot be effectively attenuated in a band out of the target band.
Further, a situation that it is necessary for occupants of motorcycles or workers in the construction site communicate with each other with sound often occurs. For example, there is a case in which the occupant of the motorcycle converses with the occupant of the nearby motorcycle at the time of stopping at an intersection or at a parking lot. It is necessary to turn off the noise attenuation system each time, which is very troublesome. Furthermore, voice is in the relatively high frequency band, and since the most of the voice is absorbed by the shock absorbing layer of the helmet, the conversation across the helmet is very difficult.